<html>
<head>
    <script type="text/javascript">

        function validate_required(field, alerttxt) {
            with (field) {
                if (value == null || value == "") {
                    alert(alerttxt);
                    return false
                }
                else {
                    return true
                }
            }
        }

        function validate_form(thisform) {
            with (thisform) {
                if (validate_required(email, "Email must be filled out!") == false) {
                    email.focus();
                    return false
                }
            }
        }
    </script>
</head>

<body>
<form action="submitpage.htm" onsubmit="return validate_form(this)" method="post">
    Email: <input type="text" name="email" size="30">
    <input type="submit" value="Submit">
</form>
</body>

</html>